As is known, the low-carbon (high-strength) low-alloy steel is one of the most important engineering structure materials, and is widely applied into oil and gas lines, offshore platforms, ship buildings, bridge structures, boiler vessels, architectural structures, automobile industries, railway transportation, and mechanical productions.
The properties of the low-carbon (high-strength) low-alloy steel depend upon its chemical components and the process system in the manufacturing process, wherein the strength, plasticity, toughness and weldability are the most important ones thereof, which finally depend on the microstructures of the finished steel product. As the science and technology develops, higher requirements on the matching of high toughness and high plasticity of high-strength steel are put forward. That is to say, the mechanical properties and operational performance can be significantly improved while maintaining a low manufacture cost, so as to reduce the amount of used steel materials, save the cost, and reduce the self-weight of the steel structure, and more importantly, to further improve the safety, stability, durability and cold/hot machinability, to accommodate different construction environments and meet different requirements on the processes.
Currently, there is a climax in research and development on a new generation of high-performance steel and iron materials in Japan, Korea and European Union. Efforts have been made to optimize the alloy combinations and innovate the manufacturing processes so as to obtain a better match among structures, such that the high-strength steel can gain a better match between high toughness and high plasticity.
The traditional thick steel plate with a tensile strength of more than 590 MPa is fabricated by reheating and quenching plus tempering (RQ+T) that is so-called “offline hardening”, which requires the central part of the steel plate to be of sufficiently high hardenability, i.e., the hardenability index DI is more than or equal to 1.0 multiplied by the thickness of the steel plate, wherein DI=0.31101/2(1+0.64Si)×(1+4.10Mn)×(1+0.27Cu)×(I+0.52Ni)×(1+2.33Cr)×(1+3.14Mo)×25.4 (mm), so as to ensure that the steel plate has sufficiently high strength, excellent ultra-low temperature toughness and uniform microstructures and properties along the thickness direction thereof. Consequently, a certain number of alloy elements such as Cr, Mo, Ni, Cu are inevitably added into the steel (JPS59-129724, JPH1-219121). Ni can not only improve the strength and hardenability of the steel plate but also reduce the phase-transition temperature and fine the grain sizes of lath bainite/martensite; more importantly, Ni is the only element for improving the intrinsic low-temperature toughness of lath bainite/martensite, increasing the orientation angle between the bainite/martensite lathes, and improving the resistance to expand cracks in the eutectic bainite/martensite. As such, the alloy content of the steel plate is high, which results in not only high production cost but also high carbon equivalent Ceq, and high welding cold crack sensitivity index Pcm. This brings large difficulties to the field welding, such that preheating is needed before welding, and heat treatment is needed after welding, whereby the welding cost becomes higher, the welding efficiency is reduced, and the welding environment becomes worse. A large number of prior patent documents (e.g. JPS63-93845, JPS63-79921, JPS60-258410, JPH4-285119A, JPH4-308035A, JPH3-264614, JPH2-250917, JPH4-143246, U.S. Pat. Nos. 4,855,106, 5,183,198, 4,137,104) describe only how to achieve the strength and low-temperature toughness of the base steel plate, but not on how to improve the welding performance of the steel plate and obtain excellent low-temperature toughness of the welding heat affected zone HAZ, nor how to ensure the hardenability of the central part of the hardened steel plate, to ensure the strength, toughness of the steel plate and the uniformity of the strength, toughness along the thickness direction thereof.
Currently, in term of improving the low-temperature toughness of the welding heat affected zone (HAZ) of the ultra-high heat input welded steel plate, only Nippon Steel Co. of Japan takes the oxide metallurgy technology (U.S. Pat. No. 4,629,505, WO 01/59167 AI), that is, during the high heat input welding process, TiN particles near the melting lines, dissolve under the strong effect of the high temperature, and fail. Ti2O3 is more stable than TiN, and does not dissolve even under the temperature of higher than the melting point of steel. Ti2O3 particles may become the nucleating sites of the austenite transgranular acicular ferrite-AF, in order to promote the nucleation thereof, divide the austenite grains effectively, fine the HAZ structure, and form high-strength high-toughness acicular ferrite-AF structures. Besides, Sumitomo Metal Co. of Japan takes the technical means of adding B, and controlling the ratio B/N higher than or equal to 0.5, low silicon, ultra-low aluminum, moderate N content, in order to solve the problem with the high heat input welding performance of 60 kg-level steel plates, which achieves good effects and has been applied to the engineering practice successfully (Iron And Steel, 1978, Vol. 64, Page 2205).